This application is a 371 continuation of PCT/EP98/04412 Jul. 16, 1997 under the International Convention and based on German Application DE 197 30 388.9 filed Jul. 7, 1997.
1. Field of the Invention
The invention relates to a contact for mechanically and/or electrically connecting a first element such as a substrate to a second element by means of a bond structure as well as to a method of producing such a contact, and the application of a corresponding contact or method, respectively.
Such contacts are employed in numerous applications; one preferred application of such contacts is the manufacture of miniaturised connections disposed in a minimum of space. The miniaturised contacts are used, for instance, in microelectronics, neuro-prosthetics or in implant medicine.
In microelectronics, for instance, it is necessary as a rule to interconnect various electronic devices such as integrated circuits, printed circuit boards, substrates, hybrids or the like. The manufacture of such mechanical, electrically conductive connections is usually performed by a method generally designated as xe2x80x9cbondingxe2x80x9d.
For the production of these connections frequently flat thin bond structures are used such as flexible, thin and narrow strip conductors which must be mechanically and hence also electrically connected to the aforementioned devices serving as substrate. Monolithically integrated circuits without a housing and also any other bondable material surfaces may be used as substrate for the assembly of such flexible structures.
In neuro-prosthetics thin flexible strip conductor structures are used which must be connected to the body for stimulating nerves or for deriving nervous signals. Like in implant medicine appropriate demands must be made on the physical compatibility of the materials used for bonding.
Further fields of application may be found wherever flexible bond structures can be employed in particular, such as in the watch-making industry or in consumer electronics, etc.
In the wake of the ever-increasing miniaturisation the spaces between the individual conductors strips and hence also between their contacting areas, i.e. the individual contacts, become ever smaller as well; for example, typical distances between the bonding contacts of adjacent strip conductors, the so-called pitches, are presently ranging at 100 xcexcm and even less.
2. Prior Art
For the production of so-called bond contacts numerous methods have become known:
In the so-called wire bonding method the connection between two devices is established by a flexible bonding wire which is fastened by means of a wire bonding device in succession on both devices (or on one device and a wiring carrier or the like). Each individual contact is fastened by pressing the bonding wire onto a contacting pad of the respective device and by soldering the wire there by means of ultra sound, thermo compression or any other weld joining technique.
As far as the feed of the bonding wire to the soldering site and the shape of its area to be soldered are concerned two methods are common in particular, the so-called ball bonding technique and the wedge bonding technique:
In ball bonding the bonding wire of the surface to be bonded is fed by means of a bonding capillary at a right angle; the end of the bonding wire which projects from a funnel-shaped flaring section of the bonding wire mount, which faces the device surface, is fused to form a sphere. This sphere fusing is normally done by means of a hydrogen micro flame or by electric spark discharge. The bonding ball so obtained is then applied onto the substrate to be bonded and soldered there in the aforedescribed manner.
The wedge bonding technique is distinguished therefrom by the fact that the bonding wire is fed at an angle of typically 30xc2x0 to 60xc2x0 relative to the contact area. Rather than being shaped to form a sphere, the free end of the bonding wire is applied below a base area on the underside of the bonding capillary, which extends approximately in parallel with the contact area of the substrate, which capillary presses the end of the bonding wire under the substrate. In this way the end of the wire piece arrives as xe2x80x9cwedgexe2x80x9d on the substrate. Soldering is equally done by means of the aforedescribed welding techniques.
As the bond wire serves to interconnect two contact sites the wire bonding technique is either referred to as ball-wedge bonding or as wedge-wedge bonding, depending on the kind of application of the first contact. In the wedge bonding technique the second contact is produced in each case by mounting an appropriately further located segment of the bonding wire, which is already fastened on one end, on the second device and by severing the bonding wire behind the fastening site.
Apart from the wire bonding techniques wherein wires are required as intermediate conductors also methods have become known for producing direct contacts between the bond structures, e.g. the so-called beam lead technique wherein flexible strip-shaped bond structures, the so-called beam leads, which project beyond the edge of the respective device, are mounted on the structures of another device.
This techniquexe2x80x94like the wire bonding techniquexe2x80x94requires, however, a huge expenditure in terms of material for the bonding material and when materials such as gold are used for bonding they involve correspondingly high production costs.
Even though smaller quantities are sufficient for other techniques in which the bond contacts are created as early as during the manufacture of the devices as such as their components, as a rulexe2x80x94for instance rigid bond mounds are modelled on at lest one of the devices in the flip chip technique, which then establish the contact when the other device is mounted in face-down positionxe2x80x94this technique is inexpedient, compared against wire bonding, however on account of the great number of required process steps and process media and as a result of the lower limit of the pitch space of some 200 xcexcm, which is due to the manufacturing process.
Moreover, the used materials such as SnPb or Cu are compatible with CMOS elements only under certain conditions, and on account of their toxicity they are not suitable for the application in neuro-prosthetics. Apart therefrom, the flip chip technique and even any other simultaneous bonding method is linked up with major interference into the manufacture of the devicesxe2x80x94lithography, coating processes, adjustment operations, etc.xe2x80x94directly so that they do not constitute a proper substitute for serial techniques such as wire bonding.
The foregoing considerations have gone to show that the common methods of producing bond contacts involve high costs whenever minute bond contacts are to be produced, which costs are due to the expenditure in terms of material and/or the complex handling or approach, respectively. Furthermore, the known methods involve restrictions also with respect to the achievable minimum pitches.
The present invention is based on the problem of improving a contact which provides a mechanical and/or electrical connection of a first part, in particular a solid background such as a substrate, with a second part such as a second substrate or a flat thin strip conductor in particular (referred to as bond structure in the following) by means of a bond connection in such a way that the two parts can be mechanically and electrically connected in the simplest way possible and in a manner saving time and costs, at a pitch as small as down to 50 xcexcm.
One invention solution to this problem is defined for a contact in and for the production method.
The inventive problem is solved for a contact by the provisions that the bond structure presents a hole, that the bond connection is substantially a bond element extending from the background through the hole in the bond structure up to a zone located above said bond structure, and that said bond element, in the area located above said bond structure, extends laterally at least partially beyond said hole in said bond structure, thus ensuring the contact of the background with the underside of said bond structure.
For the production method the problem is solved by the provision that the bond head, for most of its part, is bonded through a hole in said bond structure directly onto the background while the bond structure is pressed by the bond head against the background.
In view of the typical width of strip conductors of 100 xcexcm and less and the equally very thin thickness, the second part is, as a rule, a flat thin and flexible bond structure and presents, like the first part which may be a rigid background in particular, a contacting area suitable for bonding. The hole in the flexible strip conductor (mostly in the centre of the contacting area) allows for the simultaneous contacting of the strip conductor and the background within a minimum of space and in one step of operation. The quantity of consumed material, which is required for bonding, and the expenditure in terms of work and time is correspondingly small.
An ultrasonic welding apparatus is sufficient as equipment for carrying through the inventive method.
In this way, the cost-saving processing of fairly small numbers of units in particular becomes possible, too. Another advantage of the inventive method resides in the aspect that this method is suitable for bonding already the commercially available ICs with their standard aluminium contacts.
The mechanical contact is established by the bond element which is created by deformation of the bond head in the course of the inventive method. This bond head (in the form of a bonding ball or a bonding wedge) is larger than the hole in the bond structure. It is therefore not only pressed as such through the hole on the contacting area of the background but presses also the bond structure against the background outside the hole. As a result, the bond element is given a mushroom-shaped cross-section form in the simplest case during deformation, with the area above the bond structure surface, which corresponds to the head of the mushroom ensures a permanent mechanical contact between the bond structure and the background.
In a first inventive embodiment the feature is provided that the bond structure is conductive on its upper side in the vicinity of the hole so that it is also electrically connected, via the bond element, to the background.
In accordance with a second inventive embodiment the bond structure is conductive on its underside in the vicinity of the hole, contacts the background there, and is electrically connected to the background by this direct contact. In this case the bond element establishes the mechanical connection between the strip conductor and the background, via its area located above the bond structure, and hence ensures also the electrical connection indirectly.
With the bond material being merely pressed or formed, respectively, in the inventive method, any bondable material may be employed on principle.
According to a further embodiment gold is used as bonding material because this metal is used specifically for medical applications on account of its high chemical resistance.
An expedient embodiment of the invention provides for adjustment of the bond structure in parallel with the surface of the background by means of the hole. As the background or its contacting area, respectively, is visible through the hole in the (transparent or even opaque) strip conductor the initially required adjustment of the strip conductor prior to contacting is facilitated in this manner.
Another embodiment presents the feature that the bond head is shaped as bonding ball in a manner known per se by fusing a bonding wire. In this manner it is easily possible to enlarge the diameter of the bond head beyond the diameter of the hole so that the bond structure is covered by the bonding ball in the vicinity of the hole.